Conventional pneumatic tourniquets typically provide an inflatable cuff which may be wrapped around a patient's limb, a source of compressed gas for pressurizing the cuff, a pressure gauge for measuring the cuff pressure, and a pressure regulating mechanism. Typically, the cuff is wrapped around the patent's limb and pressurized with compressed gas to a pressure as high as 650 mmHg in order to stop the flow of blood into the limb. A surgeon is thus provided with a "bloodless field" in which surgical procedures may be performed on the limb. The pressure gauge provides the operator with an indication of cuff pressure. The pressure regulating mechanism is intended to maintain cuff pressure relatively constant.
It has been estimated that about 10,000 conventional pneumatic tourniquets are currently used in about 100,000 surgical procedures performed annually in North America. Regrettably, the wide spread use of pneumatic (and non-pneumatic) tourniquets in surgery has been accompanied by continuing reports of limb paralysis, nerve damage and other injuries believed to be attributable to tourniquets. A survey of the literature indicates that such complications may result from four factors:
1. Excessive cuff pressure (which may lead to nerve compression and other damage at the cuff site). PA1 2. Insufficient cuff pressure (which may lead to passive congestion or hemorrhagic infiltration of the nerve). PA1 3. Excessive periods of application of a pressurized tourniquet to the limb. PA1 4. Application of the tourniquet without consideration of the local limb anatomy.
Many reported cases of preventable nerve damage, limb paralysis and other injuries are believed to have resulted from the factors listed above, the most common of which appears to be overpressurization of the cuff [see: D. K. Wheeler and P. R. Lipscomb, A Safety Device for a Pneumatic Tourniquet, J. Bone Joint Surg., 45A:870, 1964; W. K. Hamilton and M. D. Sokoll, Tourniquet Paralysis, Journal of the American Medical Association, 199:37, 1967; S. J. Prevoznik, Injury from Use of Pneumatic Tourniquets, Anesthesiology, 32:177, 1970; J. M. Bruner, Time, Pressure and Temperature Factors in the Safe Use of the Tourniquet, Hand, 2:39-42, 1970; D. Fry, Inaccurate Tourniquet Gauges, Br. Med. J., 1:511, 1972; A. E. Flatt, Tourniquet Time in Hand Surgery, Arch. Surg., 104:190-192, 1972; G. Burchell and G. Stack, Exsanguination of the Arm and Hand, Hand, 5:124-126, 1973]. Unfortunately, the actual incidence of tourniquet-induced complications in surgery may not be reliably estimated because the "tourniquet paralysis syndrome" (to borrow a phrase from J. Moldaver, Tourniquet Paralysis Syndrome, Arch, Surg. 68:136-144, 1954) may be difficult to detect or may be masked by the effects of surgery, because the damage is generally transient and reversible to a large extent and because such incidents may not be consistently reported due to concern over potential legal liability. (A hospital was recently found liable for nerve injuries suffered by a patient as a result of excessive pressure applied to her arm by a tourniquet ["Hospital Liable to Patient for Tourniquet Paralysis", Citation, 38:5, Oct. 15, 1978]).
Conventional tourniquets examined by the inventor which have been linked to possible nerve injuries or paralysis associated with cuff over-pressurization have been found to have malfunctioning pressure-regulating mechanisms or inherent hysteresis in the pressure-regulating mechanism which permitted the cuff pressure to rise about 150-400 mmHg above the desired cuff pressure (which is typically in the 200-650 mmHg range). Other tourniquets have been found to have aneroid pressure gauges which produced readings inaccurate by about 200 mmHg.
Attempts have been made to reduce injuries due to cuff over-pressurization by incorporating safety features into pneumatic tourniquets. For example, rocker valves of the type used in pressure cookers have been employed as pressure relief valves [see: Wheeler and Lipscomb (supra) and Hamilton and Sokoll (supra)]. However, limitations inherent in the technology of conventional pneumatic tourniquets appear to have prevented practical implementation of significant safety features. At best, rigorous operating, inspection and maintenance programs might be implemented in an effort to ensure that conventional tourniquets are used in a manner which would minimize potential cuff over or under pressurization and detect equipment malfunctions. However, implementation of such programs would likely be labour-intensive and could occupy a significant portion of the time available to an operating room nurse or technician.
An object of the present invention is to provide a pneumatic tourniquet capable of automatically sensing and regulating cuff pressure to maintain the cuff pressure near a selected pressure (for example, within about 6 mmHg of a pressure in the 200-400 mmHg range).
A related object is to provide a pneumatic tourniquet having automatic means for sensing cuff over-pressurization and triggering an appropriate alarm. For example, the cuff over-pressurization alarm might be triggered if the actual pressure in the cuff exceeds the selected cuff pressure by more than about 15 mmHg. A closely related object is the automatic triggering of a cuff under-pressurization alarm if, for example, the actual cuff pressure falls more than about 15 mmHg below the selected cuff pressure.
Conventional tourniquets typically do not provide a means for measuring the time period for which a pressurized tourniquet has been applied to a patient's limb. A pressurized conventional tourniquet might therefore be left on a patient's limb for a dangerously long period of time simply because operating room personnel may be occupied with other matters and overlook the time. Accordingly, a further related object is to provide a pneumatic tourniquet which triggers a time alarm if the cuff is inflated for, or in excess of a selected time period which will typically vary between about 0-180 minutes.